Finally, interpreting the \(a_{i}\)  realized in an ensemble of markets as the occupied growth states in an economy lets us form a novel hypothesis: nominal rigidity is an entropic force. Entropic forces in thermodynamics are forces that have no microscopic analogy, yet have observable macroscopic effects. One of the most commonly encountered physical entropic forces is diffusionGravity may be an entropic force as well, and if it is true would actually be the best example. See [1]}.. If molecules are initially distributed on one side of a container, in short order they become uniformly distributed throughout the container. There is no microscopic force on a molecule proportional to local deviations in density \(\rho (r)\)  from average density \(\rho _{0}\)  (the volume is \(V\) )
\(F(r) \sim \rho (r) - \rho _{0}\) 
\(\rho _{0} \equiv \frac{1}{V} \int d^{3}r \rho (r)\) 
